The present application is related to the co-pending U.S. Pat. application Ser. No. 08/000,074, entitled "Apparatus and Method for Processing a Stream of Image Data in a Printing System", filed Jan. 4, 1993, the pertient portions of which are incorporated herein by reference.
The present invention relates generally to a technique for processing image data in a printing system and, more particularly to processing a stream of image data with one or more compressed bitmaps in a manner that maximizes the rate at which image data is outputted.
Electronic printing systems employ image data as the image source when producing prints. One source of image data is a document scanner which scans the documents that comprise the job and converts images on the documents to the image data necessary to produce prints. One electronic printing system capable of producing prints in this manner is referred to as the DocuTech.RTM. printing system, which is manufactured by Xerox.RTM. Corporation. In a preferred embodiment of the DocuTech.RTM. printing system, the scanner transmits bitmaps, in the form of raw (uncompressed) data to an image compression processor where the bitmap image is compressed. In conjunction with compressing the image, a prediction break table is compiled.
A format of the prediction break table is discussed in the following reference, the pertinent portions of which Encoding Standard are incorporated herein.
Title: Xerox Raster Encoding Standard ("Encoding Standard") Publication No. XNSS 178905 Publication Date: 1990 PA1 U.S. Pat. No. 5,150,462 Patentees: Takeda et al. Issued: Sep. 22, 1992 PA1 U.S. patent application Ser. No. 07/898,761 Entitled: Apparatus and Method for Multi-Stage/Multi-Process Decomposing Filed: Jun. 12, 1992
The Encoding Standard discloses a decoding algorithm which, preferably, treats an encoding section of a vector as vector binary values P.sub.encoded, and produces a vector of binary values denoted as P.sub.decoded. Data bits for P.sub.encoded are extracted in groups of 4, 8 or 12 bits. The first eight bits comprise a code SOI (start of image), and the last eight bits are a code EOI (end of image). Each scan line is decoded separately and begins with an eight-bit line boundary code (LBC), each LBC indicating which of four modes is used. Some modes of the decoder depend on previously-decoded scan lines, while others "break" the dependencies on previous scan lines. A decompression operator requires that the first scan line of an image be a break line and that break lines occur at least every 16 scan-lines. The decompression operator may specify the locations of break lines in the encoding data, using a break table that appears immediately after the encoding data. An entry in the break table describes a block of encoded data that begins with a break line. The block is described by its length, in scan lines, and the LCC for the first scan line of the block is indicated with a pointer.
Essentially, the prediction break table delineates segments of compressed image data for an image. Preferably, in the DocuTech.RTM. printing system, each bitmap can be either stored out to disk, with its corresponding break entry table, for subsequent printing at an image output terminal ("IOT") or transmitted from the image compression processor to the IOT. The IOT is coupled with a plurality of imaging channels, each of the imaging channels being adapted to decompress a plurality of segments of image data, so that, upon reading the break entry table, the imaging channels can decompress the plurality of segments of image data in parallel. As the segments of image data are decorepressed, the decompressed image data can be buffered for printing by the print engine. It has been found that this sort of parallel processing scheme serves to increase the speed of printing significantly, while minimizing costs of construction and operation.
The following reference contemplates the use of a table, in conjunction with a stored image, to facilitate display of the stored image
In a network printing system, compressed image data is transmitted to an electronic printing system in the form of a stream of data expressed in terms of a page description language ("PDL"). The PDL can include, among other things, a compressed bitmap. Preferably, as discussed in the following patent application, a decomposer, with one or more processors and suitable software, is employed to "take apart" an input document so that the PDL is "parsed" into various image related components with a preparser.
As discussed in the above-indicated reference, the pertinent portions of which are incorporated herein, the decomposer executes the PDL to generate imaging primitives. In doing this, the decomposer, for example, uses an arrangement for parsing the PDL into various image related components. The types of operations required to generate imaging primitives include, among others, binding fonts with requested fonts, any image processing on pictorial information, and/or converting line art/graphics (including bitmaps) to lower level primitives.
While the above-discussed scanning/compression scheme of the DocuTech.RTM. printing system contemplates creating a break table while compressing a bitmap, it does not contemplate a printing system which is capable of creating a break table from a PDL stream captured, "on-the-fly", from a network.